It is usual when emptying steel from metallurgical vessels to separate an impurity containing slag (the supernatant light phase) from a partly refined liquid metal (steel) below. As the flow from the vessel takes place, it is not uncommon for a funnel or vortex to be created which can entrain large amounts of slag into the flow of liquid metal with resulting metal quality problems downstream. Further, the vortex can cause corruption of a desired streamline flow of liquid steel leaving the vessel.
Steelmaking vessels such as ladles and tundishes, BOF converters and EAFs are never emptied completely in order that slag entrainment via "vortexing" and "non-vortexing" funnels be avoided or minimized. This is necessary to avoid carry-over of slag from one vessel to another and results in a loss of product quality, yield and productivity.
Flow behaviour in an emptying vessel is influenced by the rotational velocity components in the liquid. In the absence of such velocity components, liquid leaving the emptying vessel is drawn mainly from a hemi-spheroidal region surrounding the exit nozzle, and surface liquid far above the drainage nozzle shows little motion. Towards the very end of the drainage, entrainment of the supernatant fluid does occur as a "non-vortexing" funnel through a funnel-shaped core.
When significant rotational velocity components are present in the liquid, and particularly when the axis of rotation of the rotational velocity components is within close proximity to that of the axis of the drainage nozzle, a significant proportion of the drainage outflow originates from the surface of the liquid and flows downwardly. The combination of the axial flow and the rotational velocity components leads to an increase in the tangential velocity about the nozzle axis in a region close to the axis with the eventual formation of a "vortexing" funnel.
A vortex can also result when the flow across the vessel floor experiences significant flow losses caused by such factors as poor nozzle design. Where the rate of flow of liquid across the vessel floor is interrupted and decreases, some axial downward flow is inevitable and this tends to result in the formation of a vortex. Supernatant fluid entrainment then follows.
Given the inevitability of adventitious tangential velocity components resulting from filling, inert gas stirring, vessel design and the like, being present in a ladle or tundish at the beginning of teeming, the formation of a vortex and its associated funnel may account for more contamination of steel from slag than has generally been recognised before. Such a funnel will not only entrain the supernatant phase but can well result in the outflow stream being flared (i.e. non streamlined), owing to the presence therein of rotational velocity components. This flaring of the outflow stream adversely affects flow rate and, in the case steelmaking operations also results in the undesirable reoxidation of the liquid steel.
Previous devices aimed at eliminating vortices (or "vortexing" funnels) in steelmaking include castellated nozzles, floating plugs and stopper rods.
The castellated nozzle is intended to interfere with the flow of metal towards the exit nozzle, thereby tending to inhibit any rotational flows which would otherwise descend through the nozzle.
A variant of the castellated nozzle is the "ribbed" nozzle which, with a series of convex surfaces in line with the vertical axis of the outflow tend to inhibit, or at least limit, rotational flows. For a variety of reasons, these nozzles have not proved to be very effective, erosion being a major problem.
The floating plugs suffer from other disadvantages. Often the plugs do not completely shut off metal flow if the nozzle surfaces have eroded, or if the plug is not properly centered over the exit nozzle. Success rates of some 50% are typical of these plugs. By contrast, stopper rods offer an obstruction to vortexing flows that can be quite significant. It is nonetheless possible for swirling vortices to spin around an axis away from the stopper rod with attendant air or slag entrainment. Finally, stopper rods are also known to induce suction of gas from below the vessels through the drainage nozzle, thereby leading to flow instabilities, reduced flow and the possibility of reoxidation. None of these devices or techniques is completely effective in eliminating the possibility of vortexing flows.
Previous work aimed at eliminating freezing problems associated with widely-used "slide-gate" nozzle closure systems has led to the development of a variety of rotary pouring nozzles (e.g., U.S. Pat. No. 2,698,630, 3,651,998, 3,685,706, 3,760,992, 4,200,210, and 4,840,295). Such previous work teaches means to control the flow of liquid from a vessel by using an internal rotatable element within the nozzle structure so that the flow can be regulated.
U.S. Pat. No. 4,840,295 suggests that such a nozzle can reduce the likelihood of vortex formation problems but fails to recognise the parameters needed to minimise vortex formation. In a series of tests carried out using a nozzle design of equivalent geometry to that described in FIGS. 5 to 7 of U.S. Pat. No. 4,840,295, several deficiencies of the design came to light. Firstly, it was found that the presence of a supernatant "slag" phase would, even in the presence of rotational flows as low as 1 to 2 cms still lead to entrainment of the supernatant phase. From the very beginning of drainage, a significant amount of liquid was found to short-circuit its way into the drainage nozzle, leading to a rapid, funnel-like deformation of the interface between the two liquids which ultimately resulted in entrainment via a "vortexing" funnel. Secondly, the outflow stream showed considerable rotational and lateral oscillations. Thirdly, the overall discharge co-efficient of the modified nozzle was found to be significantly lower than that of a simple straight-tube nozzle of the identical exit diameter.
Such flow behaviour was prompted by the sudden acceleration of liquid at the entry ports to the nozzle, together with the significant sharp pressure drops at the respective entrances to the vertical downward nozzle, leading to the rapid entrainment and disintegration of the supernatant "slag" phase. Given the magnitude of these pressure drops along the bottom of the vessel, it was inevitable that some amount of liquid was drawn from the upper portions of the vessel (flow visualization clearly revealed the existence of helical spiral flow path-lines), and thus vortex formation and slag entrainment was inevitable. This resulted in the formation of a highly dispersed mixture of fine droplets of supernatant "slag" phase within the bulk lower "metallic" phase.
The present invention is a significant improvement over the prior art because it is designed to cause liquid exiting via the nozzle to approach the nozzle in several convergent radial streams substantially free of rotational swirl. The structure is designed to ensure that each stream travels a radial path having a length sufficient to substantially eliminate vortex entrainment, at least in the range of angular velocities normally found in steel discharge structures.
A vortex suppressing device based on the present invention can be adapted to existing metallurgical vessels without the need to modify process parameters. Additionally, the invention tends to provide a stable and compact outflow stream which is a most desirable requirement if reoxidation of the steel is to be avoided.
Accordingly, it is among the objects of this invention to address the aforementioned problems resulting from the formation of vortices or rotational flows in liquids being discharged from a container through a nozzle-like opening.
It must be ensured that the means employed to suppress downward axial flows that can lead to vortexing in a draining container should not inadvertently increase flow losses, decrease the discharge coefficient, or lead to outflow stream instabilities. Flow conditions should be so tailored as to ensure that a great proportion of the outgoing liquid is drawn radially along the vessel floor towards the nozzle, and that wherever possible very little of the surface liquid is allowed to travel through the main body of liquid to the nozzle axis.